The present invention relates to a test method and equipment to detect in-house wiring problems, for instance in an xDSL network.
The acronym xDSL stands for the family of Digital Subscriber Line technologies, which allow high-speed access to the Internet and multimedia services such as video, gaming, etc. over the local loop, which connects the CP (customer premises) to the CO (central office), typically over simple twisted pair cables. An xDSL transceiver at the CO, which is a subscriber line concentrator like a DSLAM (digital subscriber line access multiplexer) or DLC (digital loop carrier), communicates with an xDSL transceiver at the CP, which is a subscriber modem like a DSL modem or router, over the local loop. Behind the subscriber modem, in-house cabling is installed usually by the end users themselves. This in-house cabling is often not compliant with recommended installation rules. The consequence of such bad in-house wiring installation practice, is often poor performance and reliability of the xDSL link between the CO and CP: low bit rate, high bit error rate (BER), loss of data packets, loss of connection and service disruption. Such poor performing xDSL connections are not suitable for high bandwidth, real-time services like video distribution or interactive TV over xDSL, resulting in reduced revenue potential for the telecom operators.
Since decades the local loop, which is a transmission line consisting of two twisted copper wires, also called unshielded twisted pair (UTP), has given the customer access to POTS (Plain Old Telephony Service). The POTS signal, transmitted over the local loop, is analogue and contained in the frequency band up to 4 kHz, which corresponds to the spectral content of speech. xDSL exploits the frequency band above 4 kHz up to several MHz, which is not used by POTS. The frequency band above 4 kHz contains a downstream channel for transmission from the CO to the CP, and an upstream channel for transmission from the CP to the CO. The nature (e.g. asymmetric or symmetric, time domain duplexing or frequency domain duplexing, single carrier or multi-carrier, etc.) and size (achievable bit rate or capacity) of the downstream channel and upstream channel depend on the xDSL flavour: for instance ADSL (Asymmetric Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line), HDSL (High Speed Digital Subscriber Line), VDSL (Very High Speed Digital Subscriber Line), . . .
To separate the POTS and ADSL services using the same twisted pair, installation rules for instance specify that each telephone set (i.e. POTS or ISDN terminal) must mandatory be connected behind a POTS splitter containing a low pass filter for the POTS frequency band and a high pass filter for the DSL frequency band. One of the main in-house wiring problems is a missing POTS splitter. In such case, POTS phones, fax equipment, . . . and the xDSL modem or router are connected in parallel without a splitter at the right place. Some phones are transparent but a lot induce non-linear effects in the ADSL frequency band thereby reducing the performance of the ADSL connection.
Another frequently encountered problem is low quality in-house cabling. When the loop cable quality is bad, there is a lot of radio interference entering the copper pair: radio broadcasts are creating unwanted noise onto the copper pair in a narrow band fashion, which is called RFI or radio frequency interference in the DSL community. Usually, this is due to insufficient shielding and twisting of the in-house cable. Mostly in-house network suffer of this but sometimes, also the operator cable itself is of low quality, specially if aerial cabling is used like in the United States. Such RFI is affecting the ADSL performance because it reduces the available SNR (Signal Noise ratio. Hence, it reduces the amount of bits (or the data rate) that can be achieved with ADSL.
Another problem resulting from in-house wiring is impulsive noise. When using the ADSL modem for low speed Internet access, e.g. 500 kbit/s to 1 Mbit/s service, the noise margin of the modem is usually large. A large noise margin implies that very high noise peaks(ingress noise) can take place on the line without really disturbing the service. Also the TCP/IP protocol which is used to transmit Internet data over such ADSL connection, is able to repeat corrupted or lost IP packets. So, at the end the quality is always acceptable. When introducing high speed Internet access, the noise margin obviously will reduce and the transmission will be much more affected by ingress noise pulses. The problem can be such severe that so many IP packets are corrupted and need to be repeated that the user perception is a lower performance. Even worse: for video or high bandwidth real time services over Internet, the subscriber needs high data rates and there is no packet repetition possible. Any transmission error caused by
impulse noise on the line now results in a lost IP packet. In case of video, this could result in a frozen image for a duration of around 1/10 second. If this occurs too frequently, the subscriber will complain for the low quality of his video over DSL service. It is currently not possible to measure such impulse noise with the CP and CO modems synchronized at a low bit rate. The impulse noise often results from in-house wiring. Cables are twisted in some way, so there is more or less immunity to ingress noise. However, a large amount of common mode noise is captured on the line and fluctuating quite a lot over time. The in-house wiring is not perfectly well balanced and some common mode to differential mode conversion can take place. Thus, common mode noise coming into the CP modem is converted into differential noise. In the field, mainly in USA, a special type of common mode filter is used, like for instance the filter with reference L-222RJDP from the company Pulse/Excelsus. The goal is to filter out common mode signal captured along the loop before they enter the house. Impulse noise however can also be collected directly by the in-house wiring itself. This is mainly the case when the end-user places untwisted wires in-house.
Currently, detection of in-house wiring problems requires a human intervention for in-house wiring analysis. Dispatching a technician to the end-user's home to check if the in-house cabling complies with installation rules however is cumbersome and costly.
Moreover, “do-it yourself” type of xDSL installation has enabled low cost, massive deployment of xDSL connections, as a result of which telecom operator maintenance activities are unexpectedly exploding. Many xDSL end-users contact the customers care centres of these xDSL operators to complain about xDSL connection problems or low performing xDSL connections.